The present invention relates to an apparatus and a method for washing a surface of a substrate such as a semiconductor wafer or a glass substrate for an LCD device.
In the manufacture of a semiconductor device, a washing system is employed for removing contaminants such as particles, organic contaminants and metal impurities which are attached to a surface of a semiconductor wafer. In a one-by-one type wafer washing system, a semiconductor wafer supported by a spin chuck is washed with various chemical solutions while rotating the wafer, followed by rising with a pure water and, then, drying by a gas blowing.
In the wafer washing system of this type, proposed is a so-called xe2x80x9cmegasonicxe2x80x9d, i.e., ultrasonic wave of megaheltz, washing, in which an ultrasonic vibration is applied to a chemical solution so as to further improve the washing capability of the chemical solution. For example, Japanese Patent Disclosure (Kokai) No. 61-16528 discloses a megasonic washing, in which an ultrasonic wave of megahartz is applied to a chemical solution which is to be spurted from a nozzle. In the conventional megasonic washing, however, the ultrasonic vibration is attenuated within the chemical solution before the solution spurted from the nozzle reaches the wafer, resulting in failure to improve sufficiently the washing capability of the chemical solution. It should also be noted that bubbles tend to be formed within the chemical solution. If these bubbles stay within the nozzle, the ultrasonic wave fails to be transmitted to the substrate surface, resulting in failure to wash sufficiently the substrate surface.
Further, in the apparatus disclosed in JP ""528 noted above, a nozzle is scanned along a surface of the wafer so as to supply a chemical solution uniformly over the entire surface of the wafer. In this case, however, the ultrasonic wave is applied from the scanning nozzle to the wafer surface for only a short time, leading to a low through-put of the washing treatment. What should also be noted is that, in this apparatus, a large amount of a chemical solution is required for sufficiently washing the entire surface of a wafer.
An object of the present invention is to provide an apparatus and a method for washing a substrate which permit washing a substrate surface uniformly and also permit improving the through-put of the washing treatment.
According to an aspect of the present invention, there is provided an apparatus for washing a substrate, comprising a spin chuck holding and rotating a substrate, a process solution supply mechanism having a solution discharge port through which a process solution is supplied onto the substrate rotated by the spin chuck so as to form a film of the process solution,: an ultrasonic oscillator for applying an ultrasonic vibration to the film of the process solution, a relative moving mechanism for relatively moving the ultrasonic oscillator and the spin chuck so as to adjust the relative positions of the ultrasonic oscillator and the substrate, and control means for controlling each of the spin chuck, process solution supply mechanism, ultrasonic oscillator and relative moving mechanism so as to permit the ultrasonic oscillator, which extends to cover substantially a radius of the substrate, to be in contact with the film of the process solution but not to be in contact with the substrate and so as to make optimum the relationship among a gap G between the ultrasonic oscillator and the substrate, a supply amount Q of the process solution, and a rotating speed V of the substrate.
In the megasonic washing, the ultrasonic vibration is transmitted to the water molecules within the process solution so as to permit the vibrating water molecules to vibrate foreign matter such as particles and, thus, to facilitate removal of the foreign matter from the wafer surface. The process solution to which is applied an ultrasonic vibration is superior to an ordinary process solution in the washing capability.
In the present invention, the gap G between the ultrasonic oscillator and the substrate, the supply amount G of the process solution and the rotating speed V of the substrate is controlled to be optimum. As a result, the film of the process solution formed on the substrate is stabilized. In addition, the ultrasonic vibration, which is hardly attenuated, can be applied efficiently to the process solution.
During the operation, the ultrasonic oscillator, which has a length substantially equal to or slightly smaller than a radius of the substrate, is positioned slightly above the substrate not to extend across an axis of rotation of the substrate. The particular arrangement makes it possible to prevent a central portion of the substrate from being washed excessively and to wash the entire surface of the substrate uniformly.
It is desirable for the washing apparatus of the present invention to be equipped with a holder of the ultrasonic oscillator. The liquid discharge port of the process solution supply mechanism is defined within the holder so as to be positioned immediately sideward of the ultrasonic oscillator. The holder of the particular construction makes it possible to apply an ultrasonic vibration to the process solution immediately after discharge from the solution discharge port, leading to a further improved washing efficiency. Of course, it is possible to arrange separately the process solution supply mechanism and the ultrasonic oscillator.
According to another aspect of the present invention, there is provided a method of washing a substrate, comprising (a) the step of holding a substrate and rotating the substrate about an axis perpendicular to the washing surface of the substrate, (b) the step of supplying a process solution to the rotating substrate to form a film of the process solution on the washing surface of the substrate, (c) the step of arranging ultrasonic oscillating means, which extends to cover substantially a radius of the substrate, not to be in contact with the substrate but to be in contact with the film of the process solution for applying an ultrasonic vibration to the film of the process solution, and (d) the step of making optimum the relationship among a gap G between the ultrasonic oscillating means and the substrate, a supply amount Q of the process solution, and a rotating speed V of the substrate.
FIG. 7 shows the relationship among the gap G, the supply amount Q and the rotating speed V of the substrate. It is desirable to set the gap G between the ultrasonic oscillating means and the substrate to fall within a range of between 0.5 and 5.0 mm. If the gap G is smaller than 0.5 mm, collision may possibly take place between the ultrasonic oscillating means and the substrate. If the gap G is larger than 5.0 mm, however, it is difficult to form a film of the process solution in contact with both the ultrasonic oscillating means and the substrate. Where the gap G falls within the range noted above, bubbles are not formed in the film of the process solution. In addition, the process solution film can be formed stably on the substrate.
It is desirable for the process solution supply rate Q to fall within a range of between 0.2 and 2.0 liters/minute. If the supply amount Q is smaller than 0.2 liter/minute, it is difficult to wash sufficiently the substrate. If the supply amount Q is larger than 2.0 liters/minute, however, the process solution is discharged vigorously from the discharge port, giving rise to bubble formation within the film of the process solution. In addition, the consumption of the process solution is unduly increased.
The rotation speed V of the substrate should desirably fall within a range of between 40 and 180 rpm. If the rotation speed V is lower than 40 rpm it is difficult to supply the process solution uniformly over the entire surface region of the substrate. If the rotation speed V exceeds 180 rpm, however, the film of the process solution is rendered unstable and is likely to be broken.
Incidentally, the frequency of the ultrasonic wave should desirably fall within a range of between 400 kHz and 2 MHz. If the frequency of the ultrasonic wave is lower than 400 kHz, bubbles tend to be generated within the process solution by cavitation. On the other hand, if the frequency of the ultrasonic wave exceeds 2 MHz, the wafer surfaces receives a serious damage.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.